Whenever a commercial airplane crashes, officials and the public await the discovery and analysis of the “black box” flight data recorder, which indicates the status of various flight parameters over time. Often, the failure that led to disaster happened minutes or even hours before the actual crash.
Less dramatically, assume that a bag fails to turn up at baggage claim at the airport. All the owner knows, and probably all the baggage claim agent knows, is that the bag did not arrive, with no idea initially at which of the many stages of handling from initial check-in some mistake occurred. Using the bag tag identifier (typically encoded with a bar code), the airline is usually able to determine the bag's whereabouts by tracing the identifier back through a tracking system.
As useful and sometimes necessary as such fault-detection systems may be, they generally track events (aileron failure, misrouting), but the very act of initiating an inquiry, and the circumstances of the inquiry itself, may sometimes provide information as relevant, or even more relevant, than information about individual failure events. For example, knowing that there have been a large number of recent inquiries about a person's credit history might be highly relevant to a potential lender, regardless of the results of the inquiries. In short, the “metadata” about information access may sometimes be as valuable as the information itself.
Similar issues arise in the context of data, especially in environments in which data files, such as documents, may be downloaded, copied, deleted, and altered not only directly by those initially accessing them, but also by those downstream who may have gotten them from upstream users. There are several known document-control schemes for creating and maintaining access and audit logs, such that access history is preserved for later analysis, but much of the information is stored in a linear, disjointed manner that makes patterns difficult to discern, and often fail to put the data in a context informed by metadata.